Apparatus for closing and locking of die casting machines and the like



Aug. 25, 1958 u. EGGENBERGER 2,848,771

Y APPARATUS FOR CLOSING ANO LOOKING OF OIE CASTING MACHINES AND THE LIKE 5 Sheets-Sheet 1 Filed Feb. 29, 1956 QQN wm@ ONO l @QN ---,I- m N WFH %%N www NNN MN O OOO www wmm SN uWm .K1 n

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Aug. 26, 1958 u. EGGl-:NBr-:RGER 2,848,771

APPARATUS FOR CLOSING AND LOCKING OF DIE CASTING MACHINES AND THE LIKE 5 Sheets-Sheet 2 Filed Feb. 29. 1956 NNN QNN

AU8- 26, 1958 u. EGGENBERGER 2,848,771

APPARATUS FOR CLOSING AND LOCKING 0F DIE CASTING MACHINES AND THE: LIKE Filed Feb. 29, 1956 5 Sheets-Sheet 3 INVENTOR Aug. 26, 1958 u. EGGENBERGER 2,848,771

APPARATUS Foa cLosING AND LOCKING oF DIE CASTING MACHINES AND THE LIKE Filed Feb. 29, 1956 5 Sheets-Sheet 4 3559.3. pg Z504 INVENTOR /73 A'rroRNEY u. EGGENBERGER 2,848,771 APPARATUS FOR cLosING AND LOCKING oF DIE Aug. 26, 1958 CASTING MACHINES AND THE LIKE 5 Sheets-Sheet 5 Filed Feb. 29, 1956 EN N @www dk.

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@Maf IU ATTORNEY BY e.

APPARATUS FR CEL-@SUNG AND LGQKKNS @F Dill CSTNG MACHNES AND 'Il-1E LHE Ulrich flggenberger, Uzwil, Switzeriand, assignor to Gebr-nder Buhler, Uzwil, Switzeriand, a corporation of Switzerland This invention relates to apparatus requiring substantial locking forces, such as presses, die casting and injection molding machines, and more particularly to improved means for closing and locking such machines.

ln die casting machines, the die usually consists of two parts, one of which is fastened to a movable die support and the other to a stationary die supporting member. The stationary member is solidly connected with a crosshead that carries the mechanism for imparting the necessary movement to the movable die supporting member and for the generation of the locking force. The removal of the casting from the die requires a relatively large movement of the movable die supporting member. On the other hand, a very large locking force is necessary to withstand the required casting pressures.

While a great variety of mechanical and hydraulic systems for generation of the locking force are in use, they all present certain drawbacks. lf a purely hydraulic mechanism is used, a piston of large diameter is necessary which, in view of the long stroke required for opening of the die, necessitates a substantial volume of hydraulic fluid and results in slow operation of the machine. Furthermore, air bubbles in the hydraulic uid are practically inevitable and, consequently, the lock becomes very elastic and sensitive to pressure shocks during the casting operation.

The disadvantage of the mechanical locking system resides in that the mechanism requires frequent adjustment because of heat expansion, e. g., during starting and after an interruption of the operation. Furthermore, there is no control over the actual locking force. A wedge mechanism has been proposed to avoid such adjustment of the mechanical locking system. The movable die supporting member is moved by a piston of relatively small diameter and, at the end of the locking stroke, he '-.vedges that generate the locking force are pressed between the movable die supporting member and special surfaces disposed on the guide rods. To facilitate their withdrawal, it is necessary to provide the wedges with a substantial taper so that no excessive forces are necessary for their movement. The wedges must be moved while under inuence of the full locking force, which causes considerable friction and results in excessive wear and low eciency of the apparatus.

The present invention overcomes the above disadvantages of the known systems by the provision of an apparatus for generation of the die closing force directly y hydraulic pressure, and of locking force by mechanical elements.

More specically, the invention provides an apparatus wherein the movable die supporting means consists of two portions with a pressure chamber therebetween, the chamber having an inlet for introduction of hydraulic fluid under controlled pressure. Mechanical elements connect the rear portion of the movable die supporting means to the crosshead, and at least one locking wedge or locking ring is disposed between the two portions of the movable die supporting means.

Patented Aug. 26, 95e

The novel apparatus is independent of temperature changes and permits necessary movement of the closing and locking members against forces substantially smaller than the full locking force. It is actuated by a hydroelectric system requiring little hydraulic uid and permitting quick opening and closing of the die by relatively small forces.

Various other features and advantages of the claimed apparatus will become apparent in the course of the following detailed description of some at this time preferred embodiments which are illustrated in the accompanying drawing, and the invention will be pointed out in the appended claims.

In the drawing, wherein similar characters of reference indicate similar elements,

Fig. l is a side elevational view of the machine with the die shown in closed position;

Fig. 2 is a top plan view of the apparatus shown in Fig. l;

Fig. 3 is horizontal section taken on line 3 3 in Fig. 1;

Fig. 4 shows the upper half of Fig. 3, with the die in en position;

Fig. 5 is a view in the direction of arrows from line 5 5 in Fig. l, showing the wedges and the rear portion of the movable die suppe g means;

Fig. 6 is a vertical axial section taken on line 6 6 in Fig. 7, showing a modied movable die supporting means with a pair of locking wedges mounted in the pressure chamber;

Fig. 7 is a view in the direction of arrows from line 7 7 in Fig. 6, with tl e compressor piston removed;

Fig. S is a vertical axial section through a further embodiment of the movable die supporting means, taken on line 8 8 in Fig. 9;

Fig. 9 is a section taken on line in Fig. 8;

Fig. 1G is' a vertical axial section through a still further embodiment of the movable die supporting means, taken on line @G Mi in Fig. ll;

Fig. l1 is a View of the interior of the system of Fig. l0, as seen in the direction of arrows from line M ll in Fig. l0; and

Fig. l2 represents schematically the hydroelectric control system or" the machine.

Referring now in detail to the illustrated embodiments, and more particularly to that shown in Figs. l to 5, base member lil@ supports a stationary die support (hereinafter called stationary platen) 162 and a crosshead LS with four parallel guide rods 106 therebetween. Platen itil?, carries a stationary die ibi, the other half die M3 being supported by a movable die support if that is sli-fiable on the guide rods litio between the platen and crosshead ical. movable die support .lila consists of two portions E97 and iltl7aI connected by bolts iffi that permit limited displacement of said portions with respect to each other for the purpose to be more fully set forth hereinafter. The half die lo?) is carried by the front portion a (hereinafter called movable platen}. Half dies 101, M33 are suitably forrned to provide space for the casting ii (see Fig. 3). The tilling sleeve lo@ is fastened in the stationary platen itil and is provided with a bore liti extending into the casting space Mia in half die 101. The injection piston lll, actuated by a known mechanism not form'ng part of this invention and therefore not illustrated in the drawing, forces the liquid metal that is cast through the opening M" in sleeve 109 into the spaces halt .tra lilla, lla of the respective half dies 10i, 103 when the machine is closed and locked.

The movable platen li'a of the die support 104 which carries the half die is provided with a piston 115 diameter that enters the cylinder 116 in rear portion 107 moving substantially with the movable platen 107:1 (hereinafter called auxiliary platen) to forma pressurevchamber 117 with an outlet 118, as best shown in Fig. 3.

The movable platen 10711 and auxiliary platen 107 of the die support 104 are moved apart when the hydraulic fluid Vis introduced through conduit 11S into the chamber 117 within the cylinder 116. A pair of locking wedges 119 connected to double acting pistons 120 in cylinders 121 that are carried by the auxiliary platen 107 of die support 104, are slidably mounted between platens 107, 107:1 and are partially received in channels 119:1 in said platen 107. When the fluid enters cylinders 121 through conduits 123, the locking wedges 119 are forced between platens 107, 107a tending to move them apart, and are retracted when the uid enters through lines 122 of the cylinders 121. The taper or inclination of the locking wedges 119 is sufficiently small so that an axial pressure on either one of platens 107, 107:1 cannot cause a movement thereof. Stop bolts 124 that'are screwed into prog'ections 1242 on auxiliary platen 107, but are free for limited axially sliding movement in the projections 124-11: carried by the movable platen 107:1, limit the relative movement of the platens 107, 107:1.

he piston 115 may be made integral with the auxiliary platen 107 of the movable die support 104, and the Vpressure chamber is then provided in the movable platen 107:1. On the other hand, the cylinder or cylinders 121 may be attached to the movable platen 107:1.

A pair of connecting arms 125 is pivotally attached to the auxiliary platen 107 of the movable die support 104 by pins 125:1, and to a pair of shackles 126 that are in turn pivotally connected to a plate 127 attached to the rod 123:: of a double actingr piston 12S received in a cylinder 129 in the crosshead 105. The diameter of closing piston 128 is substantially smaller than the diameter of locking piston 115.

Piston 128 with plate 127 moves toward right when the pressure uid enters cylinder 129 through line 130, and is moved toward left in the cylinder 129 when the uid enters said cylinder through line 131. When the die is open, i. e., when the half dies 101, 103 are apart, as shown in Fig. 4, the arms 125 are partially retracted into the crosshead 105 in a position determined by the guide surfaces 132 in the crosshead 105 on the one hand,

and by the heels 133 of said arms that lean against the plate 127 on the other hand. When the piston 128 has completed its stroke in cylinder 129 and the plate 127 abuts against the surface 105:1 in crosshead 105, as seen in Fig. 4, the half dies 101, 103 are suticiently apart to permit the removal of the casting 108. In this open position of the half dies 101, 103, the shackles 126 are inclined with respect to the axis of the machine. It will be easily observed tha-t the shackles 126 push the arms 125 during the closing movement while the closing piston 128 and plate 127 move toward right, and will lpeil the arms 125 during the opening movement in the opposite direction.

Referring now to Fig. 3, illustrating the apparatus in closed position with the piston 128 in cylinder 129 moved `toward right, the connecting arms 125 restin recesses 134 provided in crosshead 105. The walls 134:1 of recesses 134 are nearly or fully perpendicular to the horizontal axis of the machine. When the piston 12S with plate 127 moves toward right and the locking arms 125 reach the exit end of surfaces 132 in crosshead 105, the ini clined lshackles V126 cause a pivoting movement of arms Va spreading mechanism, including shackles 126 and lock- -`ima arms 125, to either separate the half dies 101, 103

or to mechanically lock the auxiliary platen 107 of the 4 movable die support 104 with respect to the crosshead 105.

The operation of the apparatus shown in Figs. 1 to 5 is as follows:

ln Fig. 4, the die is open. Fluid is then introduced into the cylinder 129 through line 130 to displace the closing piston 128 toward right. During this movement of the piston 123, line 131 is open to permit discharge of fluid from the right-hand end of the cylinder 129. Piston rod 120:1 pushes plate 127, shackles 126, the locking aims 125, the platens 107 and 107:1 of the movable die support 104, and the half die 103 toward right. During this movement, closing piston 128 overcomes only the friction of the various moving elements and thus the force necessary for its actuation is relatively small. Because the diameter of said closing piston 128 may be made small, the movement is a relatively quick one requiring little compression uid. When the arms 125 reach the right-hand ends of guide surfaces 132 in crosshead 105, they are pivoted around pins 125:1 and .their left-hand endsl carrying heels 133 are spread by the inclined shackles 126 to enter the recesses 134 in the crosshead. Fluid is then introduced into the pressure chamber 117 in cylinder 116 through the conduit 118 to generate the locking force for the die 101, 103. The diameter of the locking piston in cylinder 116 is large and will therefore produce a considerable locking force while requiring a short movement` and very little compression uid.

Fluid is then introduced into the cylinders 121 through lines'123, while the lines 122 are open to discharge the iluid from the opposite sides of wedge actuating pistons 120. The pistonsdraw the wedges 119 tight, as shown in Fig. 3. Since the auxiliary platen 107 of the movable die support 104 is supported by mechanical elements, i. e., the arms 125 leaning against the walls 13411 in reresses 134 of crosshead 105, and because of the provision of one or more locking wedges 119 between the platens 107, 107a, the necessary locking action is provided exclusively by mechanical elements, to wit: half die` 101, stationary platen 102, guide rods 106, crosshead 10S, locking arms 125, auxiliary platen 107, locking wedges 119, movable platen 107:1 and half die 103. This system is independent from temperature changes which are compensated for by shorter or longer movement of the locking wedges 119, and the force required for such movement will remainthe same. The frequent adjustments, common in conventional mechanical locking systems without a two-piece movable die support, becomes unnecessary. It will be observed that only a fraction of the force that would be necessary if the wedges 119 were the only lockingdevices, is required in the present arrangement. An additional considerable advantage of the novel system resides in that not a single element-is moved under the influence of the full locking force, which adds to the efliciency of the machine and reduces the wear. The relatively large opening or closing movement of movable die support 104 is effected by the closing piston 128 of small diameter and the locking wedges 119 are removed or released after an additional increase in pressure in chamber 117 by the hydroelectric system shown in Fig. l2.

After the machine is thus locked, liquid metal is cast through opening 112 into the bore 110V of the sleeve 109, and is forced into the spaces 101:1, 103:1 at a substantial speed by the plunger 111, this resulting in a considerable pressure shock which is counteracted by the mechanical lock between half dies 101, 103. Upon solidification of casting 108, the pressure in chamber 117 of cylinder 116 is somewhat increased to facilitate the removal of locking wedges 119 by the pistons 120. The fluid enters cylinders 121 through lines 122 while the lines 123 are open to discharge the fluid from the opposite sides of the pis- `tons 120. Pistons displace the locking wedges 119,

whereupon the line 118 leading into the pressure chamber 117 in cylinder 116 is set to discharge to reduce the netter/vr pressure in the chamber 117 to zero. All the mechanical locking elements are now relieved and only a small force is required for their movement into the position shown 1n Fig. 4 by introducing pressure iluid through the line 131 into cylinder 129 while the line 130 is set to discharge. Piston 128 moves to left together with the plate 127 which in turn entrains the shackles 126 and locking arms 125. At the outset of movement of plate 127 toward left, shackles 126 are inclined from their substantially perpendicular position with respect to the axis of the machine (see Fig. 3) to contract the heels 133 of the locking arms 125 until these arms abut against the lateral surface of plate 127 (see Fig. 4), whereupon the arms are free to slide along guide surfaces 132 into the crosshead 105 until the plate 127 abuts against the surface 105a therein. Locking arms 125 entrain the movable die support 104 who-se platens 197, 107g become sepa rated to the extent permitted by the stop bolts 124 because of the friction of the casting 1198. The half die 103 is thereby separated from the other half die 161 and the solidified casting 1135 is removed therefrom in known manner. The ejection is achieved partly by the plunger 111, and partly by conventional ejector elements that are not represented in the drawing. Upon removal of casting 103, the plunger 111 is retracted and the machine is back in the position illustrated in Fig. 4.

In the embodiment of Figs. 6 and 7, the locking wedges 219 are mounted in the pressure chamber 217 of the cylinder 216 in auxiliary platen 207 between the cylinder wall 235 and the inclined surfaces 236 of the locking piston 21.5. This arrangement is particularly advantageous as it prevents entry of foreign matter between the cooperating surfaces of locking wedges 219, the auxiliary 207 of the movable die support 254, and the piston 215.

In the embodiment of Figs. 8 and 9, the locking piston 315 is provided with external threads 341B. The locking Wedge or Wedges are replaced 'by an internally threaded locking ring 341 having a pair of brackets 342. Piston 324) is connected to a pin 342a traversing the brackets 342 to rotate the locking ring 341 which is rotatably supported by the auxiliary platen 357 of the movable die support 304. Thus, the ring 341 assumes the function of the locking wedges in that it forces the platens 307, 307a of the movable die support 304 more or less apart, depending on the direction and extent of movement of the piston 32).

Referring now to Figs. l() and ll, the hollow externally threaded locking ring 441 is mounted within the pressure chamber 417 or the cylinder 416. A pair of flaps 442 extend radially into the bore 441z of said locking ring and are connected to the piston 420 by a pin 442:1. Locking ring 441 is axially rotatably supported by the auxiliary platen 497 of the movable die support 404. Again, the platens 467, 467g are urged apart when the locking ring 441 is rotated in proper direction by the piston 42). As seen, the internal threads 440 are provided in the bore 415a of the locking piston 415 that is integral with the movable platen 467e.

The wedges 119, 219 or locking rings 341, 441 could be actuated by other than hydraulic means, for example, by an electromagnetic system. 1t is also understood that the above described arrangement will iind application in systems other than die casting and injection molding machines, such as various types of presses and the like, where secure locking by application of considerable locking forces is required.

Finally, in Fig. l2, there is shown schematically the hydro electric system.- that controls and actuates the apparatus illustrated in Figs. l to 5. A similar system may be used to actuate either of the embodiments shown in Figs. 6, 7, Figs. 8, 9, or Figs. l0, ll, respectively.

A pump 46 in line 45 conveys the fluid from a tank 47 to the pressure regulating valve 48 which is connected with tank 47 by an overflow line 49. From valve 48, `the uid is led through conduit 51 to a four-Way valve 5 52 which is the terminal o conduits 130, 131 for actuation of the closing piston 128 in cylinder 129 attached to crosshead 105. As previously described, piston 128, rod 12811, plate 127 and shackles 126 with locking arms eect the closing or opening of the movable die support 164 and its movement with respect to the stationary platen 102. A return line 53 connects the fourway valve 52 with the tank 47.

The valve 52 is actuated by a pair of electromagnets 54, 55. Upon energizing of magnet 54, the hydraulic fluid liows from line 51 into the line 130 leading to the lett side of piston 128, with the line 131 discharging through conduit 53 into the tank 47. rl'he piston 12S with rod 12tlg and plate 127 moves toward right to close the platens 107, 197e of the movable die support 104 and to bring *he half die 193 into closing position with respect to the half die 101 (see Fig. 3).

On energizing of magnet 55, closing piston 128 is moved toward left by the pressure iiuid that enters from line 51 into conduit 131 while the line 13) is connected across valve 52 with the conduit 53 leading into the tank 47.

A conduit 57 leads from the pressure regulating valve 43 to a three-way valve 58 which is the terminal of line 11S leading into the pressure chamber 117 of cylinder 116, wherein the hydraulic uid generates the main locking force for the die 191, 103. A return line 59 from the three-way valve 55 returns the fluid to tank 47 when the halt` dies 101, 103 are to be separated. Valve 58 is actuated by a pair of electromagnets 6?, 61. After the electromagnet 6i) is energized, line 113 is connected with pressure line 57 and, inversely, the energized magnet 61 connects the line 11S with the return line 59 across the three-way valve 53.

A pressure line 33 (shown as branching oit the line 57) connects the line 57 with a four-way valve 64 that is connected to conduits 122, 123 for movement of piston 12) actuating the locking wedge or wedges 119. A return line 65 (shown as branching ott the return line 59) leading to tank 47 is also connected to valve 64, the latter being actuated by a pair of electromagnets 66, 67. On energizing of electromagnet 66, the uid is free to pass through lines 57, 63, 123 into the cylinder 121 to move piston 12th with locking wedge 119 into the position shown in dot-dash lines. The line 122 discharges through valve 64 and lines o5, 59 into the tank 47. On actuation of electrornagnet 67, the pressure iiuid is led through line o3, through valve 64 and line 122 into the cylinder 121 on the opposite side of piston 12) which consequently returns the locking wedge 119 into its position shown in full lines. l

in conduit 118, there is a reducing Valve 69 with an overflow line 7@ leading to tank 47 (shown as terminating in conduit 65). A two-way valve 71, actuated by an electromagnet 72, is installed in the overflow line 711. The reducing valve 69 may be bypassed by a line 74 wherein a two-way valve 75 is actuated by an electromagnet 76. On energizing of electromagnet 72, the overliow line 7@ is closed. When the electromagnet 72 is deenergized, the line 70 is open and the reducing valve 59 operates. Energized electromagnet 76 causes the valve 75 to open the bypass line 74, and said line is closed when the electromagnet 76 is deenergized.

Switch 31 is in the circuit of electromagnets 54, 60, 66, and a switch S2 is in the circuit of electromagnets 55, 61, 67, 72, 76. The circuit of electromagnet 65B also includes a limit switch 53` actuated by the plate 127 at the end of closing stroke of piston 128. A second limit switch 84 is installed in the circuit of electromagnet 66. This switch is actuated by the movable platen 107er of the movable die support 154 when this platen reaches the end ot its closing movement toward right. A third limit switch S5 is in the circuit of electromagnet 61 and is actuated by the wedge 119 when said Wedge reaches its dot-dash releasing position 119. Switch S5 is also connected with` electromagnet 55 over a time-lag relay 36.

The sequenceof operation of the system of Fig. l2 is as follows:

When the switch 81 is closed, electromagnet 54 causes valve 52 to permit flow of pressure uid through the conduit 1311 into cylinder 129 to move theV closing piston 128 toward right into kposition 128', whereupon the plate 127 (now in dot-dash position 127') actuates the limit switch 33, thereby sending pressure uid through the three-way valve 58 and the reducing valve 69 into the pressure chamber 117 of the cylinder 116 Via conduits 57 and 115. 5'

After. the movable platen 107e has completed itsmovement toward the stationary platen 102, as shown at 107e', it actuates the limitswitch 84 to initiate the engaging movement of the locking wedge 119. The casting operation is then completed and casting 193 allowed to cool.

Switch Slis then opened and switch S2 is closed. This energizes electromagnets 72`and 76, the latter causing valve 75 to open the bypass line 74. The pressure in cylinder 116 rises somewhat, whereupon the electromagnet 67, associated with valve 64, is energized and the locking wedge 119 disengaged, which causes actuation of limit switch 85. Switch S5 energizes the electromagnet 61 of the valve 5S to reduce the pressure in cylinder 116 to zero, and Von elapse of-atime interval determined by the time lag relay, 86, electromagnet 55 of valve 52 is energized to return the piston 128 into its opening position toward the left.

The increase in pressure incylinder 116 after the cornpletion of the casting operation is of importance as it permits removal of the locking kwedge or Wedges 119 without requiring substantial forces. It will be noted that such pressure rise, due to elements 69-76, tends to separate the two platens 1417, 117a of the movable die support 104, and a relatively small piston 126 is sufficient to retract the wedges. The reducing valve 69 is set to a pressure which is somewhat lower than the pressure in the three-way valve 5S, so that the pressure in chamber 117 of the cylinder 116-is raised when the valve 69 is bypassed by conduit 74, as previously described.

It will be seen that I have provided an improved apparatus for closing and locking the cooperating members in die casting machines, injection molding machines, presses and the like, wherein the locking action is purely mechanical and the opening and closing of the elements require little time, relatively small amounts of pressure fluid and relatively smallcylinders for quickly effecting the necessary movement. None of the locking members must be removed while under inuence of the total locking force, and the apparatus for unlocking and opening the system requires little power, is subjected to less wear and is independent of changes caused by the rictional heat. Y

Various changes and modiiications will occur to persons skilled in the art within the spirit of this invention, and l therefore do not desireV to be limited to the exact details of the apparatus shown and described, but only the' scope of the appended claims.

I claim:

l. In a die casting machine or the like having a sup; port, a stationary die platen and a crosshead rigidly mounted on said support, a movable die platen between said stationary platen and said crosshead, and means for guiding said movable platen between said stationary platen and said crosshead, the combination of an auxiliary platen between said crosshead and said movable platen` means for connecting said movable platenwith said auxiliary platen and permitting limited movement of said last named Vplatens with respect to one another, said movable platen and said auxiliary platen defining a pressure chamber of relatively large cross-sectional area therebetween, `a supply of hydraulic fluid, a conduit for connecting said supply of uid with said chamber, a source of, pressure in said conduit, mechanical elements operatively connected with said crosshead and said auxiliary platen for moving said auxiliaryplaten and said movable platen toward and away from said stationary platen and for locking said auxiliary platen with respect to said crosshead, a cylinder supported by said cross- K head, a iirst double acting piston 'in said cylinder, an operative connection between said source of pressure and said cylinder, means for connecting said mechanical elements with said rst piston, and atleast one locking member between said auxiliary platen and said movable platen for rigidly retaining said movable platen with respect to said auxiliary platen.

2. The structure of claim l, further including means in said conduit for varying the pressure of hydraulic tluid in said chamber.

3. The structure of claim l, wherein said means for varying the pressure of hydraulic fluid in said chamber include a reducing valve in said conduit between said source of pressure and said chamber, and a second conduit including la control valve for bypassing said reducing valve in advance of release of said locking members.

4. The structure of claim l, wherein said locking members are mounted in said chamber.

5. The structure of claim l, further including a second double acting piston for each of said locking members, said second pistons being operatively connected with said locking members for actuatingA the'latter, and an operative connection is provided between said source of pressure and said second pistons for actuating same.

6. The structure of claim 5, wherein said locking members are Wedges.

7. The structure of claim 5, wherein kthe cross-sectional area of said chamber is greater than the combined crosssectional areas of said rst and said second pistons.

8. The structure of claim l, wherein Yone locking member is provided between said auxiliary platen and said movable platen, said locking member being a threaded ring attached to one of said platens, the other of said platens having a threaded portion meshing with the threads of said ring, whereby said platens maybe locked with respect to one another when said ring is rotated by said second piston in one direction, and unlocked when said ring is rotated in the opposing direction.

9. The structure of claim 8, wherein said ring is internally threaded and rotatably attached to said auxiliary platen, said movable platen having an externally threaded portion meshing with the internal threads in said ring.

10. The structure of claim 8, wherein said chamber is provided in said auxiliary platen, said movableplaten having a third piston extending into said chamber, said third piston having a threaded bore, said ring being eX- ternally threaded, rotatably attached to said auxiliary platen and meshing with the threads in said bore.

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